1. Field of the Invention
This invention relates to a process for determining the lithology of and type of fluid within a subterranean formation by means of a neutron generation source and a gated gamma ray detection device.
2. Description of the Prior Art
Efficient exploitation of a potential oil-bearing field requires information concerning the lithology and fluid content of the subterranean formations which are penetrated by the oil well. One technique used extensively in the petroleum industry to ascertain this information is radioactive well logging.
One method of radioactive well logging is to bombard a desired formation with primary radiation, normally neutrons, and observe the induced instantaneous or secondary radiation, normally gamma rays. The number and energy of the induced secondary radiations are characteristic of the presence and amounts of various isotopes of the chemical elements contained in the formation fluids and matrix. Thus an induced nuclear radiation log may be employed to determine the presence and amounts of carbon, for example, in subterranean formations. This type of log does not log directly for desired materials, such as oil or sulfur. Instead characteristics, such as the chemical nature of the subterranean formation and its fluids, are determined from which the presence and amounts of any desired material are calculated. The application of these methods, however, is not accurate in all cases and when applied to certain types of the formations may yield ambiguous results. For example, the relative amount of induced gamma rays from carbon in a water-saturated limestone may be as great as those from carbon in an oil-saturated sandstone.
A well logging method which can overcome the problems associated with the prior art techniques is needed to more accurately determine the lithology of and the type of fluids within subterranean formations.